Solid state imaging device having transition time relationship for drive signals

ABSTRACT

A CMOS sensor has unit pixels each structured by a light receiving element and three transistors, to prevent against the phenomenon of saturation shading and the reduction of dynamic range. The transition time (fall time), in switching off the voltage on a drain line shared in all pixels, is given longer than the transition time in turning of any of the reset line and the transfer line. For this reason, the transistor constituting a DRN drive buffer is made proper in its W/L ratio. 
     Meanwhile, a control resistance or current source is inserted on a line to the GND, to make proper the operation current during driving. This reduces saturation shading amount. By making a reset transistor in a depression type, the leak current to a floating diffusion is suppressed to broaden the dynamic range.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a solid-state imaging device arrangedwith a plurality of unit pixels so that a signal can be desirablyselected and read out of the individual unit pixel under addresscontrol, and to a control method for the same. More particularly, theinvention relates to a solid-state imaging device of a type having unitpixels each structured by a photoelectric converter element and threetransistors without having a select transistor, and to a drive controlmethod for the same.

2. Description of the Related Art

The amplification-type solid-state imaging device (APS: Active PixelSensor, also called a gain cell) as one kind of X-Y addressingsolid-state imaging device is structured with pixels by the use ofactive elements (MOS transistors) such as the MOS structure, in order toprovide the pixel with an amplification function. Namely, the signalcharge (photoelectrons), stored on the photodiode as a photoelectricconverter element, is amplified by the active element and read out asimage information.

In the X-Y addressing solid-state imaging device of this +kind, a pixelregion is constituted, for example, by arranging a multiplicity of pixeltransistors in a two-dimensional matrix form. The storage of a signalcharge corresponding to incident light is commenced line by line (row)or pixel by pixel so that the current or voltage signal based on thestored signal charge can be sequentially read out of the pixels byaddress designation.

Conventional Unit-Pixel Structure: First Example

FIG. 19A shows a first example of conventional unit pixel 3. The unitpixel 3 of the first example is of a 4-transistor structure generallyemployed as a CMOS sensor, as broadly known in the conventional.

The unit pixel 3 of the first example has a charge generating section 32having a photoelectric converting function to convert light into chargeand also a charge storing function to store the charge, a read selecttransistor 34 as an example of a charge reading section (transfer gatesection/read gate section) with respect to the charge generating section32, a reset transistor 36 as an example of reset gate section, avertical select transistor 40, and an amplifier transistor 42 in asource-follower configuration as an example of a detector element todetect a potential change at a floating diffusion 38.

The read select transistor 34 is to be driven by a transfer drive buffer150 through a transfer line (read select line) 55. The reset transistor36 is to be driven by a reset drive buffer 152 through a reset line 56.The vertical select transistor 40 is to be driven by a select drivebuffer 154 through a vertical select line 52.

Meanwhile, the unit pixel 3 has a pixel-signal generating section 5 inan FDA (floating diffusion amp.) structure with a floating diffusion 38as an example of charge injecting section having a function of chargestoring part. The floating diffusion 38 is a diffusion layer having aparasitic capacitance.

The reset transistor 36 of the pixel-signal generating section 5 has asource connected to the floating diffusion 38, a drain connected to thepower source VDD, and a gate (reset gate RG) to which a reset pulse RSTis to be inputted from the reset drive buffer 152.

The vertical select transistor 40 has a drain connected to the powersource VDD, a source connected to the drain of the amplifier transistor42, and a gate (particularly referred to as a vertical select gate SELV)connected to the vertical select line 52. To the vertical select line52, a vertical select signal is to be applied. The amplifier transistor42 has a gate connected to the floating diffusion 38, a drain connectedto the source of the vertical select transistor 40, and a sourceconnected to the vertical signal line 53 through the pixel line 51.

In this arrangement, the floating diffusion 38 is connected to the gateof the amplifier transistor 42. Accordingly, the amplifier transistor 42outputs a signal commensurate with the potential at the floatingdiffusion 38 (hereinafter referred to as an FD potential) onto thevertical signal line 53 through the pixel line 51. The reset transistor36 resets the floating diffusion 38. The read select transistor(transfer transistor) 34 transfers the signal charge generated by thecharge generating section 32 to the floating diffusion 38. Amultiplicity of pixels are connected to the vertical signal line 53. Inorder to select a pixel, the selected pixel only is turned on at itsvertical select transistor 40. Thereupon, the selected pixel only isconnected to the vertical signal line 53, to output a signal of theselected pixel onto the vertical signal line 53.

In this manner, the unit pixel 3 is generally structured with thevertical select transistor 40 for the purpose of pixel selection. Theunit pixel 3 in most today's CMOS sensors possesses a select transistor.

Conventional Unit-Pixel Structure: Second Example

Contrary to this, there is a proposal of a unit pixel 3 structured by aphotoelectric converter element and three transistors as shown in FIG.19B (hereinafter referred to as a second example of unit pixel 3), as atechnology for reducing the pixel size by decreasing the area thetransistor occupies within the unit pixel 3 (see Patent Document 1, forexample)

[Patent Document 1]

Japanese Patent No. 2708455

The unit pixel 3 of the second example has a charge generating section32 (e.g. photodiode) for generating signal charge commensurate with thelight received by photoelectric conversion, an amplifier transistor 42connected to the drain line (DRN) and for amplifying a signal voltagecorresponding to the signal charge generated by the charge generatingsection 32, and a reset transistor 36 for resetting the chargegenerating section 32. Meanwhile, a read select transistor (transfergate section) 34, to be scanned by a not-shown vertical shift registerthrough the transfer line (TRF) 55, is provided between the chargegenerating section 32 and the gate of amplifier transistor 42.

The gate of the amplifier transistor 42 and the source of the resettransistor 36 are connected to the charge generating section 32 throughthe read select transistor 34 while the drain of the reset transistor 36and the drain of the amplifier transistor 42 are connected to the drainline. Meanwhile, the source of the amplifier transistor 42 is connectedto the vertical signal line 53. The read select transistor 34 is to bedriven by a transfer drive buffer 150 through a transfer line 55. Thereset transistor 36 is to be driven by a reset drive buffer 152 throughthe rest line 56. The transfer drive buffer 150 and the reset drivebuffer 152 are both to operate on two values, i.e. reference voltage 0 Vand power voltage. Particularly, the low level voltage to be supplied tothe gate of the conventional-example read select transistor 34 in thepixel is 0 V.

In the unit pixel 3 of the second example, the floating diffusion 38 isconnected to the gate of the amplifier transistor 42 similarly to thefirst example. Accordingly, the amplifier transistor 42 outputs to thevertical signal line 53 a signal commensurate with the potential at thefloating diffusion 38.

The reset transistor 36 has a reset line (RST) 56 extending in the rowdirection and a drain line (DRN) 57 provided common between most of thepixels. The drain line (DRN) 57 is to be driven by a drain drive buffer(hereinafter referred to as a DRN drive buffer) 140. The resettransistor 36 is to be driven by the reset drive buffer 152, to controlthe potential at the floating diffusion 38. Here, the techniquedescribed in Patent Document 1 has the drain line 57 separated in therow direction. The drain line 57 has to flow a signal current in anamount of the pixels of one row, hence being actually a line commonbetween all the rows in order to flow current in the column direction.

The signal charge generated by the charge generating section 32(photoelectric converter element) is transferred to the floatingdiffusion 38 by the read select transistor 34.

Here, the unit pixel 3 of the second example is not provided with avertical select transistor 40 connected series with the amplifiertransistor 42, differently from the first example. Although the verticalsignal line 53 is connected with a multiplicity of pixels, pixelselection is effected not by a select transistor but under control of FDpotential. Usually, the FD potential is kept low. When selecting apixel, the FD potential is raised high at a selected pixel, to send asignal of the selected pixel onto the vertical signal line 53.Thereafter, the FD potential at the selected pixel is returned to a low.This operation is effected simultaneously on the pixels of one row.

For FD potential control in this manner, operations are made, i.e. 1)when the FD potential on the selecting row is made high, the drain line57 is raised to the high to thereby raise the FD potential to the highthrough the reset transistor 36 on the selected row, and 2) when the FDpotential on the selected row is returned to the low, the drain line 57is made low to thereby decrease the FD potential to the low through thereset transistor 36 on the selecting row.

However, the present inventor has trial-manufactured a solid-stateimaging device structured by the second type of unit pixels 3, andthereby recognized the problems, i.e. 1) the characteristic is differentbetween the pixel at the periphery and the pixel at the center, thuscausing shading phenomenon wherein, particularly, the maximum charge(saturation electrons) to be stored on the photoelectric converterelement is less at the center, and 2) dynamic range is narrow.

In respect of the two problems, the present inventor has analyzed thephenomena and revealed the followings.

-   -   1) The drain line 57 is a line extending through nearly the        entire of the pixel region. When driving it, there occurs        potential fluctuation on the well (hereinafter, explanation        continued with a P-type well) of the pixel region. Although the        contact for giving a potential to the P-well is placed around        the pixel region, the manner of P-well potential fluctuation        differs depending upon a distance from the contact, thus causing        variations in the pixel characteristic. Particularly, when the        drain line 57 is made low, the P-well is biased toward the        negative, thereby leaking a signal charge from the charge        generating section 32 to the floating diffusion 38 and P-well.        At the center located distant from the contact of the P-well,        the P-well potential fluctuation is greater to thereby lose        saturation electrons at the center. This is called saturation        shading.    -   2) Following the period of driving the selected-row pixel and        reading a signal therefrom (H ineffective period), there is a        period to sequentially output the signal to the outside (H        effective time). During the H effective period, in the case of        driving to maintain the drain line 57 high, the FD potential        gradually rises due to the leak current through the reset        transistor 36. Due to this, because the difference decreases        between the selected row and the non-selected row, dynamic range        is delimited and narrowed.

The two problems and analysis results are the new matters not to existon the CMOS sensor of a type to select every pixel by the verticalselect transistor 40.

The present invention has been made in view of the above situation, andit is a first object to provide a drive technique capable of improvingthe shading phenomenon, particularly the saturation shading phenomenon,as encountered in using a device having unit pixels in athree-transistor structure.

Meanwhile, the present invention has been made in view of the abovesituation, and it is a second object to provide a drive techniquecapable of improving the reduction of dynamic range resulting from theleak current from the reset transistor, as encountered in using a devicehaving unit pixels in a three-transistor structure.

SUMMARY OF THE INVENTION

A drive control method of the present invention is a drive controlmethod for a solid-state imaging device having unit pixels structured byincluding a charge generating section for generating a signal chargecommensurate with a received light and three transistors. The drivevoltage to the drain line is driven by being blunted such that the offtransition time on a voltage waveform in driving the drain line islonger (preferably, in a range of five times or greater and ten thousandtimes or smaller, more preferably 50 to 600 times) than the offtransition time on a voltage waveform in driving any of the reset lineand transfer line.

A first solid-state imaging device according to the invention is asolid-state imaging device arranged for implementing the foregoing drivecontrol method of the invention. The drive voltage to the drain line canbe driven by being blunted such that, when a drive pulse is applied tothe drain drive buffer, the off transition time on a voltage waveform indriving the drain line is longer (preferably, in a range of five timesor greater and ten thousand times or smaller, more preferably 50 to 600times) than the off transition time on any of the reset line to bedriven by a reset drive buffer and the transfer line to be driven by thetransfer drive buffer. Incidentally, the transition time of a drivepulse itself to be applied to each drive buffer may be made as a “pulse”in a general concept having a fall and rise fully shortened.

The arrangement, in which off transition time on the drain linesatisfies the foregoing condition, can employ various contrivances. Forexample, the transistor connected to the drain line may have a W/L ratioset in a range of 1/5 times to 1/2500 times, preferably 1/10 times to1/500 times, of any of the W/L ratio of the transistor connected to thetransfer line and the W/L ratio of the transistor connected to the resetline (corresponding to the improve technique based on the first approachin the embodiment; the first example, referred later).

Meanwhile, a resistance element for limiting a drive current may beprovided between an off reference line of the drain drive buffer and areference line regulating an off voltage to the drain line(corresponding to the improve technique based on the first approach inthe embodiment; the second example, referred later). By adjusting thevalue of the resistance element, the foregoing condition can befulfilled. Also, by adjustment, setting is possible to the optimal statereduced in saturation shading. With a variable resistance value, thearrangement is more preferable.

Meanwhile, a current source for regulating a drive current may beprovided between an off reference line of the drain drive buffer and areference power source regulating an off voltage to the drain line(corresponding to the improve technique based on the first approach inthe embodiment; the third example, referred later). With a variabledrive current, the arrangement is more preferable.

Meanwhile, a second solid-state imaging device of the invention is asolid-state imaging device having unit pixels structured by including acharge generating section for generating a signal charge commensuratewith a received light and three transistors wherein, by making negativethe low level voltage of the gate of a read select transistor, apotential barrier is formed to prevent the charge from leaking from thecharge generating section to the charge storing part (corresponding tothe improve technique based on the second approach in the embodiment,referred later). Incidentally, the voltage is given in a magnitude ofrange not to cause breakdown. Meanwhile, the off voltage is preferablyset with a negative voltage in a magnitude capable of generating a holechannel at an interface of a semiconductor structuring the unit pixels.With a variable setting voltage, the arrangement is more preferable.

A third solid-state imaging device of the invention is a solid-stateimaging device having unit pixels structured by including a chargegenerating section for generating a signal charge commensurate with thereceived light and three transistors, wherein unit pixels are arrangedproviding a bias line to fix the potential on the well formed of asemiconductor (corresponding to the improve technique based on the thirdapproach in the embodiment, referred later). In this case, it ispreferred to provide each unit pixel with a contact connecting betweenthe bias line and the well.

A fourth solid-state imaging device of the invention is a solid-stateimaging device having unit pixels structured by including a chargegenerating section for generating a signal charge commensurate with thereceived light and three transistors, wherein a reset section forresetting a signal charge at the charge storing part is structured by atransistor in a depression type (corresponding to the improve techniquebased on the fourth approach in the embodiment, referred later). In thiscase, the transistor of the reset section is preferably to set thecharge storing part at a resetting level of nearly a voltage level onthe active drain line.

A fifth solid-state imaging device of the invention is a solid-stateimaging device comprising: an imaging region arranged with a pluralityof pixels; and a circuit region for supplying a drive pulse to theimaging region; the pixel having a photoelectric converting section forgenerating a charge commensurate with an amount of incident light, acharge storing part for storing a charge read from the photoelectricconverting section by a transfer gate section, and a reset section forresetting the charge stored at the charge storing part; wherein thetransfer gate is connected with a transfer line, the reset section isconnected with a reset line and the charge storing part is connectedwith a drain line through the reset section; the circuit regionsupplying a first pulse toward the drain line, a second pulse toward thereset line and a third pulse toward the transfer line; the first pulsehaving a waveform longer in off transition time than a waveform of thesecond pulse and third pulse.

The present invention has been made by conducting an analysis on theproblem of the unit pixel in a three-transistor structure noted in theprior art, and finding a resolution technique (operation principle andthe effect) to be detailed later in the embodiment.

For example, the technique based on the first approach has been made byfinding points, i.e. saturation shading is reduced by increasing thetransition time (e.g. fall time in switching over to the low, on theNMOS sensor) in turning off the voltage to the drain line common to allthe pixels, the optimal value thereof exits under certain device drivecondition, and so on. For example, applied is a technique of makingproper the W/L ratio of a transistor structuring the drive buffer ormaking proper the operation current during drive by inserting a controlresistance or current source between reference voltages. Due to this,the transition time in turning off the drain voltage is increased longerthan the transition time in turning off any of the reset line and thetransfer line, preferably five times or greater and ten thousand timesor smaller. This makes uniform the characteristic between the pixels atthe periphery and at the center, thereby reducing saturation shading.

Meanwhile, the technique based on the second approach can make thepotential barrier high relative to the charge storing part by deepening(switching toward the negative, if on the NMOS sensor) the electrodevoltage during turning off the read select transistor than a masterreference voltage (e.g. GND level) regulating the reference voltage overthe pixel entirety, thereby reducing saturation shading.

Meanwhile, the technique based on the third approach has been made byfinding the point that the fluctuation of well potential can besuppressed by providing a bias line for fixing the well potential,making it possible to reduce shading.

Meanwhile, the technique based on the fourth approach has been made byfinding the point that, by making in a depression type the resettransistor of the reset section structuring the unit pixel, leak currentis suppressed to a charge storing part such as the floating diffusionthereby broadening the dynamic range at the charge storing part.

Incidentally, the improvement technique based on the first to fourthapproach can be applied in a desired combination without limited tosingle application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic arrangement diagram of a CMOS solid-state imagingdevice according to an embodiment of the present invention;

FIG. 2 is a figure explaining saturation shading phenomenon;

FIG. 3 is a figure showing a measuring circuit for examining a voltagechange on the drain line;

FIG. 4 is a figure showing a relationship between resistance value andsaturation shading when the DRN control pulse is blunted;

FIG. 5 is a figure showing the result shown in FIG. 4 as a relationshipbetween voltage-waveform fall time on the control resistance andsaturation shading;

FIG. 6 is a figure showing a result of reproducing P-well fluctuation bysimulation (control resistance 146=0Ω);

FIG. 7 is a figure showing a result of reproducing P-well fluctuation bysimulation (control resistance 146=10Ω);

FIG. 8 is a figure showing a result of reproducing P-well fluctuation bysimulation (control resistance 146=150Ω);

FIG. 9 is a figure showing a result of reproducing P-well fluctuation bysimulation (control resistance 146=330Ω);

FIG. 10 is a figure showing a result of reproducing P-well fluctuationby simulation (control resistance 146=680Ω);

FIG. 11 is a figure explaining a first example of fall time controlmethod according to an improvement technique based on the firstapproach;

FIG. 12 is a figure explaining a modification to the technique realizingthe fall time control method in the first example;

FIG. 13 is a figure explaining a second example of fall time controlmethod according to an improvement technique based on the firstapproach;

FIG. 14 is a figure explaining a third example of fall time controlmethod according to an improvement technique based on the firstapproach;

FIG. 15 is a figure showing a relationship between transfer gate lowlevel and saturation shading;

FIG. 16 is a figure explaining an improvement technique based on thesecond approach;

FIG. 17 is a figure explaining an improvement technique based on thethird approach;

FIG. 18 is a figure explaining an improvement technique based on thefourth approach; and

FIG. 19 is a figure showing a structural example of a unit pixel in theconventional CMOS sensor.

DETAILED DESCRIPTION OF THE INVENTION

Now embodiments of the present invention will be explained in detailwith reference to the drawings. Note that the below exemplifies theapplication to a CMOS imaging device, an example of an X-Y addressingsolid-state imaging device. Meanwhile, the CMOS imaging device isexplained on the assumption that all the pixels are configured by theNMOS.

<Solid-State Imaging Device Arrangement>

FIG. 1 is a schematic arrangement diagram of a CMOS solid-state imagingdevice according to an embodiment of the present invention. Thesolid-state imaging device 1 is applicable as an electronic still cameracapable of imaging for color pictures. For example, in a still-pictureimaging mode, setting is made to a mode for reading all the pixels.

The solid-state imaging device 1 has an imaging section arranged, onrows and columns, with pixels (i.e. in two-dimensional matrix form)including light-receiving elements to output a signal commensurate withthe amount of incident light, providing an signal output, or voltagesignal, from each pixel. Namely, this is a column type having CDS(correlated double sampling) process functioning sections on therespective columns. Specifically, as shown in FIG. 1A, the solid-stateimaging device 1 has a pixel section (imaging section) 10 arranged witha plurality of unit pixels 3 on rows and columns, a drive controlsection 7 provided at the outside the pixel section 10, and a CDSprocessing section (column circuit) 26. The drive control section 7 hasa horizontal scanning circuit 12 and a vertical scanning circuit 14, forexample.

Although FIG. 1A shows rows and columns by omitting part thereof inorder for simplicity, actually several tens to several thousands ofpixels are arranged on each row or column. Meanwhile, the drive controlsection 7 has another constituent element, i.e. a timing generator (oneexample of read-out address control unit) 20 for supplying a pulsesignal in predetermined timing to the horizontal-scanning circuit 12,vertical-scanning circuit 14 and CDS processing section 26. Thoseelements of the drive control section 7 are integrally formed, togetherwith the pixel section 10, on a semiconductor region of single-crystalsilicon or the like by the use of a similar technique to thesemiconductor integrated circuit fabrication art. Thus, those arearranged to constitute a solid-state imaging element (imaging device),an example of a semiconductor system. The unit pixels 3 in the pixelsection 10 are connected to the ground (GND), a master reference voltagethat defines the reference voltage for the device entirety.

Incidentally, the timing generator 20 may be provided as a separatesemiconductor integrated circuit, independently of the other functioningelements such as the pixel section 10 and the horizontal scanningcircuit 12. In this case, an imaging device is architected by an imagingdevice having a pixel section 10, a horizontal scanning section 12 andthe like, in combination with a timing generator 20. This imaging devicemay be provided as an imaging module built with a peripheral signalprocessing circuit, a power supply circuit and so on.

The unit pixels 3 are connected to the vertical scanning circuit 14through vertical control lines 15 for selecting a vertical column and tothe CDS processing section 26 through vertical signal lines 19,respectively. Here, the vertical control lines 15 represent theinterconnection overall extending from the vertical scanning circuit 14to the pixels. For example, in the pixel of FIG. 19B, where a transferline 55, a reset line 56 and a drain line extend from the verticalscanning circuit 14, the drain line also is included. The horizontalscanning circuit 12 and the vertical scanning circuit 14 are configuredincluding for example a decoder, to start a shift operation (scanning)in response to the drive pulse given from the timing generator 20.Consequently, the vertical control line 15 includes various pulsesignals (e.g. reset pulse RST, transfer pulse TRF, and DRN control pulseDRN) for driving the unit pixel 3.

The CDS processing section 26 is provided as a column circuit on eachcolumn, to receive a signal in an amount of the pixels of one row andprocess the signal. For example, this carries out a process to take adifference between a signal level (noise level) and a signal levelimmediately after resetting the pixel, on the voltage-mode pixel signalinputted through the vertical signal line 19, on the basis of two samplepulses SHP and SHD supplied from the timing generator 20. This removesnoise signal components, called fixed pattern noise (FPN) and resetnoise. Incidentally, in the rear stage to the CDS processing section 26,an AGC (auto gain control) circuit and ADC (analog digital converter)circuit can be provided as required in the same semiconductor region asthe CDS circuit section 26.

The horizontal scanning circuit 12 has a horizontal decoder 12 a forregulating a horizontal column to be read (selecting an individualcolumn circuit of within the CDS processing section 26) and a horizontaldrive circuit 12 b for guiding the signal of each CDS processing section26 onto a horizontal signal line 18 according to a read address asdefined by the horizontal decoder 12 a. The vertical scanning circuit 14has a vertical decoder 14 a for regulating a vertical row to be read(selecting a row of the pixel section 10) and a vertical drive circuit14 b for supplying a pulse onto and driving the control line to the unitpixels 3 on the read address (in row direction) as defined by thevertical decoder 14 a. Incidentally, the vertical decoder 14 a is toselect a row for electronic shutter, etc., besides a row to read out asignal. The timing generator 20 outputs a horizontal address signal tothe horizontal decoder 12 a and a vertical address signal to thevertical decoder 14 a. In response, the decoder 12 a, 14 a selects thecorresponding row or column.

The voltage signal processed by the CDS processing section 26 isconveyed to the horizontal signal line 18 through a not-shown horizontalselect switch driven on a horizontal select signal from the horizontalscanning circuit 12. The signal is further inputted to an output buffer28, and thereafter supplied as an imaging signal SO to an externalcircuit 100. Namely, in the column-type solid-state imaging device 1,the output signal (voltage signal) from the unit pixel 3 is outputtedthrough the vertical signal line 19, the CDS processing section 26, thehorizontal signal line 18 and the output buffer 28, in the order. In thedriving, a pixel-output signal in an amount of one row is sent parallelto the CDS processing sections 26 through the vertical signal lines 19while the CDS-processed signal is outputted serial through thehorizontal signal line 18. The vertical control lines 15 are for controlin selecting a row.

Incidentally, as long as driving is available based on each verticalcolumn or horizontal row, there is a freedom as to whether the pulsesignals are arranged in the row direction or in the column directionwith respect to the unit pixel 3, i.e. how to make a physicalinterconnection of drive clock lines for applying pulse signals.

As referred later, it is preferred to supply a negative voltage to thepixel through the vertical drive circuit 14 b. For this reason, there isa possible case to mount a negative voltage generating circuit.Naturally, negative voltage may be externally supplied instead ofmounting the same.

The external circuit 100 for the solid-state imaging device 1 takes acircuit configuration corresponding to the picture-taking modes. Forexample, as shown in FIG. 1B, provided are an A/D (analog to digital)converter section 110 for converting the analog photographic signal SOoutputted from the output buffer 28 into digital photographic data DO,and a digital signal processing (DSP) section 130 for carrying out adigital signal processing based on the photographic data DO digitized bythe A/D converter section 110. The digital signal processing section 130carries out a color-separation process and generates image data RGBrepresentative of R (red), G (green) and B (blue) images, and makesanother signal process on the image data RGB thereby generating imagedata D2 for output onto the monitor. Meanwhile, the digital signalprocessing section 130 has a functioning section to make a signalcompression process in order to save photographic data onto a recordingmedia.

Meanwhile, the external circuit 100 has a D/A (digital to analog)converter section 136 for converting the image data D2 digital-processedby the digital-signal processing section 130 into an analog image signalS. The image signal S1 outputted from the D/A converter section 136 isforwarded to a not-shown display device, such as a liquid-crystalmonitor. The operator is allowed to make various operations whileviewing a display image on the display device.

The unit pixel 3, although omitted of showing in detail, is configuredsimilarly to the three-transistor structure shown in FIG. 9B concerningthe prior art. The drain line 57 is common between almost all the pixelsof the pixel section 10, which is an interconnection extending in thecolumn direction to have a common end at the end of the pixel 10 or in agrid form opened by a hole over a charge generating part 32. The pixelsmay pertly include those the drain line 57 is separated, e.g. dummypixels. Meanwhile, around the pixel 10, there is provided aninterconnection and contact for providing a potential to the P-wellthough omitted of showing.

Because the drain line 57 is connected to almost all of or all thepixels, potential fluctuation occurs on the P-well of the pixel 10 whenswitching the drain line 57 over to the low. Because there is differencein fluctuation amount and time between at the periphery and at thecenter, there is at the center an increase of the leak charge from thecharge generating part 32, to decrease saturation signal charge at thecenter. Namely, as explained in the prior art, unless measure is taken,the first problem is encountered, i.e. the characteristic differsbetween the pixels at the periphery and at the center.

FIGS. 2 to 10 explains the first problem as well as the first techniqueas a countermeasure approach to the same. At first, FIG. 2 explainsconcretely the first problem (saturation shading phenomenon). The deviceuses a CMOS sensor conforming to the VGA rating on approximately 0.3million pixels (640×480 pixels). The unit pixel 3 is thethree-transistor structure as shown in the second example in the priorart, having a pixel pitch of 4.1 μm. Note that VGA is an abbreviation of“Video Graphics Array” that defines graphics modes and displayresolutions.

The power voltage to be supplied to a trial-manufactured device isassumably 3.0 V with a clock frequency of 6 MHz (frame rate 13.3 fps).The trial-manufactured device is made to variably change the low levelof transfer gate drive voltage (hereinafter, referred also astransfer-gate low level). Furthermore, it has a terminal DRNL(ground-sided terminal of the DRN drive buffer) for supplying a lowlevel potential (0 V, herein) to the drain line 57. The other driving tothe unit pixel 3 is on 0 V (ground: GND) and a power voltage (3.0 V).

In the measuring method for saturation shading, while radiation isapplied in an amount to fully cause saturation in the charge generatingpart, the signal obtained in the output buffer 28 was measured, as ashading amount, a difference between at the periphery and at the centerby observing one line nearby a screen center by means of a waveformmonitor such as an oscilloscope. As shown in FIG. 2, on thetrial-manufactured device used in verification, a great difference isobserved between at the periphery and at the center. It can be seen thatsignal output is smaller at the center than that at the periphery.

FIG. 3 shows a measuring circuit for examining a voltage change on thedrain line 57. A control resistance 146 was inserted between theground-sided terminal DRNL of the DRN drive buffer 140 as atrial-manufactured device and the GND, to measure a voltage across thecontrol resistance 146. The voltage source 149 was at 0 V. Incidentally,the DRN drive buffer 140 is provided within the vertical drive circuit14 b of the vertical scanning circuit 14.

When a DRN control pulse (pulse-formed DRN control signal) is inputtedto the DRN drive buffer 140 (not shown) of the trial-manufactureddevice, the voltage waveform measured on the control resistance 146reflects a waveform of the current flowing the DRN drive buffer 140,representing also a voltage waveform on the drain line 57. Note that thecontrol resistance 146 closely relates to the second example ofimprovement technique based on the first approach while the voltagesource 149 to the improvement technique based on the second approach.

FIG. 4 shows a relationship between resistance value and saturationshading in the case the control resistance 146 is inserted between theground-sided terminal DRNL and the GND to thereby blunt the DRN controlpulse. In the figure, the low level voltage at the transfer gate isshown by Vt1. The resistance values used in measurement were 1, 10, 47,150, 330, 680 and 1000 (unit: Ω) conforming to the E12 system.

As shown in FIG. 4A, at Vt1=−0.6 V, saturation shading less changes in arange of 1-10 Ω wherein a sign of change in saturation shading is seenat around 10 Ω and greater. A great change is observed at 50 Ω andgreater. Namely, in case the control resistance 146 has a value smallerthan approximately 10 Ω, there is no affection upon the device in thepresent situation. Meanwhile, if it is increased to approximately 10 Ωor greater, there is obtained an effect to reduce saturation shading. At50 Ω or greater, significant effect is obtainable.

Meanwhile, as shown in FIG. 4B, at Vt1=−1 V, a great change insaturation shading is observed also in the range of 1-10 Ω. It reachesthe minimum in a range of 50-200 Ω. In the higher range than that (e.g.200Ω-nearly 1000 Ω), there is a tendency toward the somewhat increase ofsaturation shading. Namely, there is observed a significant effect toreduce saturation shading at a resistance value of approximately 10 Ω orgreater of the control resistance 146. It is considered best at around50-200 Ω.

FIG. 5 is the result shown in FIG. 4 shown as a relationship betweenvoltage-waveform fall time (off transition time) and saturation shadingat the control resistance 146. The drive pulse form for the CMOS sensoris usually set with a fall and rise time (on transition time) of severalns (e.g. 1-3 ns) on any of those on the transfer line 55, the reset line56 and the drain line 57. Accordingly, in case the voltage waveform falland rise time appears of nearly several ns or smaller on the controlresistance 146, it may be considered the device is being drivengenerally under the usual condition.

As shown in FIG. 5A, at Vt1=−0.6 V, saturation shading less changes upto a fall time of 10 ns (equal to or nearly 3-10 times greater than theusual) corresponding to the range of 1-10 Ω. A sign of change is seen ataround 10 ns corresponding to approximately 10 Ω. A great change isobserved at 40 ns and greater corresponding to approximately 50 Ω.Namely, if noticing the fall time, there is no affection upon the devicein the present situation at around 10 ns or smaller. Also, at greaterthan nearly 10 ns, there is obtained an effect to reduce saturationshading. At 40 ns or greater, significant effect is obtainable. Thiseffect is sustained up to a fall time of 10000 ns (equal to or smallerthan nearly 3000-10000 times the usual).

Meanwhile, as shown in FIG. 5B, at Vt1=−1 V, a great change insaturation shading is observed also in a range of 10-40 ns. At a falltime of 40 ns (nearly 13-20 times greater than the usual) or greater, asignificant effect is seen. Particularly, saturation shading reaches theleast in a range of 170-600-1000 ns (nearly 56-1000 times greater thanthe usual) corresponding to a resistance value of nearly 50-200 Ω. Inthe greater range than that (e.g. up to 1000-5000 ns, nearly 330-5000times greater than the usual), there is a tendency toward a somewhatincrease of saturation shading. Namely, saturation shading can beimproved by blunting the DRN voltage. At a fall time of nearly 40 ns orgreater, observed is a significant effect to reduce the saturationshading. It can be considered best at nearly 170-600 ns (e.g.approximately 56-600 times greater than the usual).

In this manner, effective range differs depending upon the low levelvoltage Vt1. However, saturation shading can be improved by blunting theDRN voltage in a range of off transition time (fall time in thisembodiment) of from 3-10 (nearly 5 times in average) times to 10000 (tenthousands) times that of the usual, preferably in a range of nearly50-600 times.

FIGS. 6 to 10 show a result of the P-well potential fluctuationreproduced by simulation. These figures are shown based on the value ofcontrol resistance 146. Incidentally, the reason of showing a simulationresult here is because of the difficulty in actually measuring a P-wellpotential fluctuation. The waveform lines W1-W4 in the figure are atrespective device positions shown in the figure. Meanwhile, as shown inFIG. 6, the waveform line SEL_0 represents a characteristic on the DRNpulse while the waveform line VSS_D is a characteristic at the terminalmeasured in the experiment.

As is shown, it can be seen that, when the value of control resistance146 is increased, the fall time of DRN voltage increases on the drainline 57, to decrease the P-well potential fluctuation and also decreasethe difference between at the center and at the periphery. Namely,increasing the value of control resistance 146 and the fall time of DRNvoltage leads to a characteristic uniform on the peripheral pixel and onthe center pixel. It can be seen that this is highly effective forimproving the saturation shading.

The present embodiment has an arrangement for improving saturationshading by blunting the DRN voltage on the basis of the above analysisresult, as an improvement technique based on the first approach foreliminating the first problem (saturation shading phenomenon).Specifically, a contrivance is provided for improving the saturationshading phenomenon by taking control of the fall time during switchingthe drain line 57 over to the low. Briefly explaining the contrivance,employed is a drive method of increasing the fall time to allow slowrise when switching the drain line 57 over to the low. This can reducethe amplitude of P-well potential fluctuation or decrease the P-wellpotential difference between at the periphery and at the center of thepixel region 10. In the present embodiment arrangement, the fall time issignificantly (intentionally) increased greater than that of the usualdriving technique.

Concerning the definition technique for increasing the fall time greaterthan that in the usual driving technique, various definition techniquescan be considered including a method of regulation with a percentage(multiple) to a drive pulse fall time in the usual drive technique, amethod of regulation with a ratio of fall time corresponding to thenumber of pixels (specifically, drive period), or a technique ofdefining a time the potential difference between at the periphery and atthe center of the P-well is equal to or smaller than a predeterminedlevel (in level image quality deterioration is not conspicuous).

Meanwhile, in the case of regulation by means of a percentage (multiple)to a drive pulse fall time in the usual drive technique, regulation maybe by a comparison with the other drive pulse without limited to thecomparison with the DRN-voltage fall time in the own's usual driving.For example, the buffer for driving the line may be determined in itssize such that DRN-voltage fall time is a predetermined multiple or morelonger than a fall time of any of the transfer and reset lines.

Meanwhile, the restoration operation of a selected pixel to anon-selected state is made by reducing the DRN control pulse to a lowlevel during the blanking period. In the case of regulation by means ofa fall time percentage corresponding to the drive period, how to definethe maximum value is problematic. It is preferred to define the maximumvalue by a low-level period of DRN control pulse for example, and todefine an actual fall time within the range thereof. With the CMOSsensor of the present experiment, the DRN control pulse has a low levelperiod (i.e. off period on the drain line 57) set at approximately 600ns.

Incidentally, the above is not to exclude the setting of a fall timeequal to or greater than the off period on the drain line 57. In thepresent experiment, the fall time of equal to or greater than 600 ns wasdetermined from an extrapolation curve of measurement data. In thiscase, it is required to attain a low voltage for restoring the selectedpixel to a non-selected state.

In any case, there has been found the problem that characteristic isdifferent between the pixels at the periphery and at the center, whichis responsible for the P-well potential difference. In order toeliminate the problem, the improvement technique based on the firstapproach is characterized in that a fall time is set to a level at whichthe image-quality deterioration (saturation shading phenomenon)resulting from a P-well potential difference is not conspicuous.

For example, based on the result shown in FIGS. 2 to 10, based on theresult shown in FIGS. 2 to 10, a fall time is provided 10 times or morelonger than a fall time of each of the transfer pulse TRF and resetpulse RST as the other pulses for the pixel region 10. For example,although the pulse form in the other section of the CMOS sensor has afall time of nearly several ns, this is made such that the DRN voltageon the drain line 57 is greater than 40 ns (nanoseconds). This 40 ns isa half duration of the pixel clock period when an image is outputted at30 frames per second from the CMOS sensor at VGA (approximately 0.3million pixels). Although here is shown the case with the CMOS sensorconforming to VGA, it is considered that those having other displayresolutions are applicable provided that they are greater than a halfduration or more of the pixel clock period.

Where display resolution, or total pixel count, is different, naturallythe absolute value of fall time is also different correspondingly.Incidentally, fall time may be on the general definition, i.e.transition time of from 90 to 10 provided that the high level is taken100 while the low level is 0. In the below, explanation is made on aconcrete case of an improvement technique based on the first approachfor eliminating the foregoing saturation shading problem.

Improvement Technique Based on the First Approach: First Example

FIG. 11 explains a first example of a method for controlling a fall time(fall-time control method) of the drive voltage to be applied to thedrain line 57. Here, FIG. 11A is a concept view noticing the circuit fordriving the drain line 57, FIG. 11B is a diagram showing a detailedexample of the DRN drive buffer (hereinafter, referred merely to asbuffer) 140 to drive the drain line 57, and FIG. 11C is a figure showingone example of drive timing.

As shown in FIG. 11A, the drain line 57 extend in the column directioncorrespondingly to the column of the pixel region 10, to have a lowerend connected to an output terminal of the DRN drive buffer(hereinafter, referred merely to as buffer) 140. The buffers 140 areprovided on the respective columns, to which a control pulse (DRNcontrol pulse) for driving the drain line 57 is applied externally ofthe pixel region 10. In response, the buffers 140 equally drive thedrain lines 57 on the columns. Namely, the drain line 57 on each columnis common on all the pixels.

As shown in FIG. 11B, the buffer 140 on the drain line 57 is configuredusing two stages of CMOS type inverters 142, 144. The inverters 142, 144are configured by an NMOS transistor shown at reference-a and a PMOStransistor shown at reference-b, respectively. The both transistorscollectively are also called a buffer transistor. Here, usually thefinal-staged inverter 144 connected to the drain line 57 has anincreased buffer-transistor W/L ratio (W: gate width, L: gate length),not to increase the both of rise time and fall time. For example, asshown in FIG. 11A, for a pixel-count VGA class on the arrangement havingbuffers 140 on the columns of drain lines 57, fall time is providedseveral ns or less. Consequently, the buffer final-staged NMOStransistor 144 a is conventionally set with a W/L ratio of, for example,approximately 5-10/0.6 (typically 10/0.6).

On the contrary, in the arrangement of the first fall-time controlmethod, the buffer transistor W/L ratio is made smaller than that of theusual (conventional) arrangement thereby positively (intentionally)increasing fall time. Particularly, fall time is intentionally increasedwithout increasing the W/L ratio of the NMOS transistor 144 a. Forexample, as contrasted with the foregoing (pixel-count VGA class in theFIG. 11A arrangement), setting is preferably at nearly 1/0.6-1/20.

Namely, setting is made significantly small relative to that of theconventional arrangement. For example, in the above example, setting ispreferably in a range of nearly 1/10-1/320 in terms of the usual ratio.Naturally, this is a mere one example, wherein setting is preferably atleast in a range of nearly 1/5-1/500.

Incidentally, definition herein was in comparison with the bufferfinal-staged W/L ratio in the conventional arrangement. However, becausethe transistors for driving the transfer line (read select line) 55 inthe unit pixel 3 and reset line 56 must be made several ns or less inrise or fall time, the foregoing numerical relationship is true for incomparison with the W/L ratio of those. Namely, the transistor connectedto the drain line 57 preferably has a W/L ratio set in a range of1/5-1/500, preferably in a range of nearly 1/10-1/320, relative to theW/L ratio of any of the transistor connected to the transfer line 55 andthe transistor connected to the reset line 56. In the usual design,there is no possibility to set the buffer final-staged W/L ratio at sucha small value.

By doing so, as shown in FIG. 11C, although the DRN control pulse to beapplied to the buffer 140 has a fall time of nearly several ns or less,the DRN voltage outputted from the buffer 140 and driving the drain line57 has a pulse form having a fall time of 40 ns or greater. This cansolve the problem that saturation electrons are less at the center ofthe pixel region 10, reduce the P-well potential difference down to thepractical level, and improve the image-quality deterioration ofsaturation shading.

Incidentally, there is no affection of saturation electrons upon therise side. However, this is similar to the fall in that potentialfluctuation occurs in the P-well and moreover this is different betweenat the periphery and at the center. Meanwhile, although not confirmed onthe trial manufacture by the present inventors, where there is alow-voltage N-type diffusion layer within the pixel, it acts as a P-welland forward bias. There is a fear that electrons injected into theP-well are to enter the charge generating part 32. Accordingly, thebuffer final-staged (i.e. inverter 144) PMOS is preferably fabricatedsmall to make also the rise slow if time is available. However, becausepixel signal current flows at the high level of the drain line 57, thereis a need for such a degree as not to cause a problematic voltagereduction.

Improvement Technique Based on the first Approach: Modification To thefirst Example

FIG. 12 is a figure explaining a modification to the technique forrealizing the first example of fall-time control method. Here, FIG. 12Ais a concept view noticing the circuit to drive the drain line 57, FIG.12B is a figure showing a detailed example of a DRN drive buffer 140 fordriving the drain line 57, and FIG. 12C is a chart of the W/L ratiobased on the first example of fall-time control method arranged incomparison with the prior art.

As shown in FIG. 12A, this modification is characterized in that thedrain line 57 is driven laterally of the pixel region 10. The pixelregion 10 has left and right ends connected with output terminals of thebuffers 140. The drain line 57 is a grate-formed interconnection openedby a hole over a photodiode (charge generating part 32). The buffers 140are provided on the respective rows. The buffers 140 drive equally thedrain lines 57 on the rows, according to a DRN control pulse of from theexternal of the pixel region 10. Incidentally, herein, explicitly drawna buffer final-staged lower power-source line, i.e. only a sourceterminal of an NMOS transistor 144 b. This line is made as a GND line,as shown in FIG. 12B.

In this manner, for the arrangement the drain lines 57 are to belaterally driven wherein fall time is given several ns or less for thepixel-count VGA class, fall time is provided several ns or less.Conventionally, the buffer final-staged NMOS transistor 144 b has a W/Lratio set, for example, at nearly 5-10/0.6 (typically 6/0.6), similarlyto the arrangement having the buffers 140 on the columns.

On the contrary, this modification sets the NMOS transistor 144 b with aW/L ratio at nearly 1/1-1/20. Namely, setting is preferably in a rangeof nearly 1/10-1/200, relative to that of the conventional arrangement(in the usual ratio). Naturally, this is mere one example, e.g. settingis preferably at least nearly 1/5 or less to 1/300 or less.

In the usual design, there is no possibility to set the bufferfinal-staged W/L ratio at such a small value. By doing so, the pulseform for driving the drain line 57 is given a fall time of 40 ns orgreater, similarly to the showing in FIG. 11C. This makes it possible toenjoy the similar effect to that of the arrangement having buffers 140on the columns.

Incidentally, the first example of fall-time control method can adopt anarrangement that the drain lines 57 over the pixel-region entire surfaceare to be driven by one DRN drive buffer 140, besides the arrangementsshown in FIGS. 11A and 12A. This arrangement, although not to bepractically employed in the usual design, can be adopted in the presentfirst example. In this case, when adopting the conventional contrivancewith a fall time of several ns or less, the buffer final-staged NMOS isset with a W/L ratio of approximately 5000/0.6, for example. On thecontrary, in the case that the contrivance based on the present firstexample is employed, a fall time is provided 40 ns or greater by settingthe W/L ratio of the NMOS transistor 144 b at nearly 500/0.6-2/0.6 (in arange of nearly 1/10-1/2500 in terms of the usual ratio).

As explained above, according to the first example of fall-time controlmethod, the transistor configuring the buffer for driving the drain line57 has a W/L ratio set smaller than that of the usual (conventional)arrangement. This can positively (intentionally) increase the fall timeon the drive voltage. Due to this, even on the three-transistor type ofpixel structure not having a select transistor, it is possible toprevent the reduction of saturation electrons at the center of the pixelregion due to potential fluctuations in the well. As a result, it ispossible to reduce the image-quality deterioration resulting from P-wellpotential difference to a level not practically conspicuous, thusimproving image quality.

Improvement Technique Based on the First Approach: Second Example

FIG. 13 explains a second example of fall-time control method. Here,FIG. 13A is a concept view noticing the circuit for driving the drainline 57 while FIGS. 13B, 13C and 13D show modifications to the secondexample.

The second example technique is characterized in that the resistanceelements for limiting the drive current are inserted between a lowerpower line (off reference line) of the drain line 57 and buffer 140(particularly buffer final stage) for driving the same and a low-levelvoltage source (reference power source defining an off voltage for thedrain line 57, including GND).

The drive circuit has a basic arrangement same as that of the firstmodification shown in FIG. 12A. The difference lies in that the bufferfinal-staged lower power line, i.e. source terminal of the NMOStransistor 144 b of the inverter 144, shown explicitly extended is notdirectly connected to the GND line but connected to the GND line throughthe control resistance 146.

Meanwhile, the figure conveniently shows the final-staged lower powerline straightly extended for the buffers 140 on the vertical one column.Also, although omitted of showing, the DRN drive buffer 140 for drivingthe drain line 57 is similar in detail to that of the first modificationshown in FIG. 12B. The second example technique is similarly applicableto the arrangement shown in FIG. 11A and arrangement for driving thedrain lines over the pixel region entirety by one DRN drive buffer.

According to the second example technique, even in case the buffertransistor W/L ratio is given great as in the usual, the use of thecontrol resistance 146 can increase the fall time during switching thedrain line 57 over to the low, similarly to the first example technique.Accordingly, this can enjoy the effect to improve saturation shading,similarly to the first example technique.

In the technique of adjusting only the transistor W/L ratio as in thefirst example technique, the W/L ratio if determined in design is not tobe modified easily. On the contrary, with the second technique,resistance value can be changed by merely changing only one fabricationmask. Otherwise, it is possible to take an arrangement that, as shown asa modification in FIG. 13B, a plurality of resistance elements arepreviously provided (resistance switchover circuit) so that a resistanceelement can be selected by an internal program (may be selected in adesired combination). In this case, it is very easy to change resistancevalue. Incidentally, the control resistance 146 and the resistanceswitchover circuit may be provided outside the device.

As can be seen from the device analysis shown in FIGS. 2 to 10, theexperiment gives a preferred result not problematic in operation speedwhile preventing the reduction of saturation electrons at thepixel-region center where control resistance 146 uses a value ofapproximately 50-200 Ω at a pixel count of VGA class. The modificationshown in FIG. 13B if applied can conveniently find a suitable valueunder actual device conditions and set such a suitable resistance value.

Incidentally, the second example technique is not limited to theapplication to the construction shown in FIG. 13, i.e. the device shownin FIG. 12A but to the device shown in FIG. 11A similarly. Meanwhile,the arrangement is not limited the insertion of the control resistance146 at one point to the GND, but may insert it together with the buffer140. In this case, the arrangement may be by an insertion to between theground-sided line terminal of each buffer 140 and the GND as shown inFIG. 13C, by an insertion to the output side of each buffer 140 as shownin FIG. 13D, or in a combination of these. The arrangement of FIG. 13Dis effective in moderating not only the fall but also the rise. Thearrangement of insertion to the source side of each buffer 140 is todistribute the control resistance 146 to the buffers 140, which issubstantially equivalent to the arrangement shown in FIG. 13A.

Improvement Technique Based on the First Approach: Third Example

FIG. 14 explains a third example of fall-time control method. Here, FIG.14A is a concept view noticing the circuit for driving the drain line 57while FIG. 14B shows one example of drive timing.

The third example is characterized in that a current source forregulating the drive current is inserted between a lower power line (offreference line) of the drain line 57 and buffer 140 (particularly bufferfinal stage) for driving the same and a low-level voltage source(reference power source regulating an off voltage for the drain line 57,including GND). Specifically, the control resistances 146 used in thesecond example technique are replaced with a current source 148. Thisarrangement is equivalent to an arrangement that the low-level voltagesource (corresponding to the voltage source 149 shown in FIG. 3) isreplaced with the GND. The third example technique is applicablesimilarly to the arrangement shown in FIG. 11A or the arrangement ofdriving all the drain lines on the pixel-region entire surface by oneDRN drive buffer.

The fall time over the drain line 57 can be controlled on the currentunder control of the current source 148. The current source 148 may beconfigured by inserting one N-type transistor or by current control bymeans of a current mirror. In brief, it is satisfactory to maintain acurrent flow nearly constant. Various configurations are applicable. Byregulating the driving current value, the foregoing condition can besatisfied and setting is possible to the optimal state reduced insaturation shading. With the set current variable in value, theconfiguration is provided more preferable. The constant current sourceis the usual one as in the above. No constant current is allowed to flowat around 0 V. In FIG. 14B, the curve is moderated to settle at 0V.

In the foregoing first and second example technique, DRN potential(output voltage of buffer 140) suddenly drops initially of fall, asshown in FIG. 11C. On the contrary, the third example techniquesuppresses (regulates) DRN potential from suddenly dropping over theentire duration of fall. Accordingly, similarly to the first and secondexample technique, it is possible to increase the fall time duringswitching the drain line 57 over to the low. Thus, the effect to improvesaturation shading can be enjoyed.

<Improvement Technique Based on the Second Approach>

Now a second approach for saturation-shading improvement is explainedfrom a viewpoint different from the improvement technique based on thefirst approach shown in the first to third examples.

FIG. 15 shows a relationship between transfer-gate low level andsaturation shading. The measurement condition is under that the controlresistance 146 has a resistance value of 0 Ω (ground-sided terminal DRNLis connected to the GND without providing the control resistances 146).

As shown in FIG. 15A, it can be seen that the absolute value ofsaturation shading decreases at a transfer gate level of approximately−0.7 V and smaller. Meanwhile, as shown in FIG. 15B, the shading amountrelative to an end saturation signal, i.e. the ratio of shading,decreases when the transfer-gate low level is given negative. It can beseen that it reaches constant at approximately −0.8 V.

The improve technique based on the second approach, noticing this point,uses an arrangement allowing a transfer-gate low level to set a negativevoltage for forming a potential barrier against the charge leak from thecharge generating section 32 to the floating diffusion 38 (chargestorage point).

FIG. 16 explains an improvement technique based on the second approach.The transfer drive buffer 150, already mentioned in FIG. 19B, has alevel shifter 160 and an output buffer 161, to output an input pulsehaving a low level GND as a pulse having a low level in negativevoltage. The negative voltage is supplied from an incorporatednegative-voltage generating circuit 162. The negative-voltage generatingcircuit 162 may be a usual charge pump circuit. Naturally, a negativevoltage may be supplied from the external instead of incorporating thenegative-voltage generating circuit 162.

By making negative the low level (Vt1) of the transfer transistor gatevoltage, saturation electrons can be suppressed from decreasing at thepixel-region center. This is because, by making negative the low levelvoltage Vt1, the potential barrier can be heightened against the chargeleak from the charge generating section 32 to the floating diffusion 38.Note that the maximum value in minus is given in such a degree as not tocause breakdown on the device.

As was shown in FIG. 15, according to the experiment, the ratio ofshading relative to saturation signal amount is decreased by making thelow level voltage Vt1 negative. This technique is allowed to workindependently of the improvement technique based on the first approachfor blunting the DRN voltage shown in the first to third examples. Ascan be seen from FIG. 15, the arrangement is made further preferable bya variable setting voltage value.

The relationship figure of between low level voltage Vt1 and saturationshading, shown in FIG. 15, was to observe the effect of low levelvoltage Vt1 without blunting the potential on the drain line 57. In thisfigure, the absolute value of shading amount is smaller than 0 V at −0.7V or smaller. At −0.8 V or smaller, both saturation signal amount andshading amount are constant. This is because, at −0.8 V or smaller, ahole channel reverse in polarity to signal charge is caused at anSi-oxide film interface (Si-semiconductor interface) constituting theunit pixel 3 whereby, even if the low level voltage Vt1 is loweredfurthermore, there is merely a change of hole concentration in thechannel without causing a change in bulk state. Such phenomenon iscalled pinning phenomenon. Accordingly, if based on the aboveexperiment, it is preferred to set the voltage source 149 with an outputvoltage of approximately −0.7 V or smaller. More preferably, it isdesired to provide a sufficient value (e.g. approximately −0.8 V) orsmaller for causing a hole channel at the semiconductor interface.

Incidentally, the present inventors have proposed, in JP-A-2001-6657, atechnology that the low level voltage Vt1 is made as a negative voltage,for the unit pixel 3 structured with four transistors including a selecttransistor. In this respect, this is common to the technique describedin the fourth example. However, the technique in JP-A-2001-6657 aims atreducing dark current whereas the forth example technique aims atsuppressing the phenomenon that saturation voltage decreases at thecenter of the pixel region 10 for the unit pixel 3 consisting of 3transistors. Thus, the objects are different from each other. Namely,the phenomenon handled by the fourth example technique is unique topixel selection through the reset transistor 36 by switching the DRNpotential wherein the unit pixel 3 does not include a select transistorconnected in series to an amplifier transistor 42. The fourth examplestructure can suppress the saturation-shading problem unique to thethree-transistor structure by making negative the low level voltage Vt1to the unit pixel 3.

<Improvement Technique Based on the Third Approach>

Now, explained is an improvement technique based on a third approach.The third approach is characterized in an arrangement that the unitpixel 3 has an interconnection for fixing well potential. Specifically,the well potential is fixed by the use of a bias interconnection andcontact (well contact) for providing a potential to the P-well.

FIG. 17 explains the improvement technique based on the third approach.Herein, shown is a concept diagram noticing the unit pixel 3. In eachpixel 3, a P-well bias line 59 for providing a potential to the P-wellis arranged parallel with a vertical signal line 53 within the pixel, asshown in FIG. 17. In each pixel 3, a p-well contact (hereinafterreferred also to as a well contact) 59 a is provided as an example ofcontact connecting between the p-well bias line 59 and the well, at apredetermined point of the p-well bias line 59. This structure cansuppress the fluctuation amount and time of P-well potential, thusmaking uniform the characteristic between the pixels at the peripheryand at the center. Namely, it is effective in improving the saturationshading to apply the technique of suppressing the affection of P-wellpotential fluctuations with the use of the P-well contact 59 a.

The contrivance based on the third approach is desirably implemented inplace of taking the countermeasure of the first or second approach.Naturally, it may be combined with the countermeasure technique of thefirst or second approach.

For the pixel of a four-transistor structure having a select transistorallowing for a large-sized pixel, there have been a case that a wellcontact is provided within the pixel. However, it is apparent that noserious problem is encountered in the absence of a well contact, fromthe fact that no well contact is provided within the pixel on most CMOSsensors now released or commercially manufactured. Naturally, it is notprovided in the improvement technique contrivance based on the firstapproach.

However, in the three-transistor structured unit pixel 3 omitted of theselect transistor, the select transistor is omitted for the purpose ofreducing the pixel size. The provision of the well contact 59 a withinthe pixel runs counter to the pixel size reduction as shown FIG. 17. Forthis reason, if considering the usual design approach, there is lesspossibility to select a structure applying a well contact to thethree-transistor structure.

Nevertheless, the provision of a well contact 59 a within the pixelshown in FIG. 17 makes it possible to prevent against the phenomenonthat saturation electrons decreases at the pixel-region center uniquelyto the tree-transistor type omitted of the select transistor.Furthermore, the improvement technique based on the third approachoffers great effect in that the area is required less than that having aselect transistor.

Incidentally, although the illustrated example is prepared with a P-wellcontact 59 a in each unit pixel 3, it is in order to prevent against theoccurrence of image unevenness resulting from well voltage unevenness.Where such event is to be permitted, the P-well contacts 59 a may bedispersed in arrangement point, e.g. every several pixel, withoutlimited to every pixel.

<Improvement Technique Based on the Fourth Approach>

Now, explained is an improvement technique based on a fourth approach.The fourth approach is characterized in that the reset transistor 36constituting the unit pixel 3 is made as a depression type, therebyeliminating the dynamic range reduction unique to the tree-transistortype having no select transistor. The circuit arrangement of the unitpixel 3 may be the same as that applied in the foregoing approach,wherein the difference is only in the device structure for use as areset transistor 36.

First, explained is the problem of dynamic range reduction. In case thedrain line 57 is continuously kept low, there is a case that electronsleak from the drain line 57 to the charge generating section 32(photoelectric converter element) by way of the floating diffusion 38thus resulting in noise occurrence. Consequently, the drain line 57 isheld high in the horizontal effective period occupying most time.However, even in case the reset transistor 36 is off, electrons passfrom the floating diffusion 38 onto the drain line 57 due to the leakcurrent through the reset transistor 36, raising the potential at thefloating diffusion 38. Particularly, there is confirmed a phenomenonthat, particularly in low-speed operation, the potential at the floatingdiffusion 38 increases higher by 100 to 400 mV than the initial, withinone frame.

The three-transistor type unit pixel 3 having no select transistor makesuse of the nature to select the highest one in the potential at thefloating diffusion 38 from among a plurality of pixels connected to thevertical signal line 53. In the selected pixel, the floating diffusion38 is reset to high level and then the signal charge (photoelectrons) ofthe charge generating section 32 is transferred to the floatingdiffusion 38. Thereupon, FD potential is changed toward the lower.

Accordingly, when the FD potential rises on the non-selected pixel,there is decrease in the potential difference from the selected pixel,making it impossible to take a dynamic range. The phenomenon of dynamicrange reduction is unique to the pixel of three-transistor type havingno select transistor. The improvement technique based on the fourthapproach uses the depression type of the reset transistor 36 in order toavoid such dynamic range reduction.

FIG. 18 explains the improvement technique based on the fourth approach.Here, FIG. 18A is a timing chart of drive pulse. Meanwhile, FIGS. 18Band 18C are a voltage potential chart.

In operation of the selected pixel, the floating diffusion 38 is set athigh level by a first reset pulse (RST). Then, with a transfer pulse(TRF), signal charge is introduced into the floating diffusion 38,reducing the potential at the floating diffusion 38. It is the readcondition that, at this time, the FD potential at the selected pixel ishigher than the FD potential at the other pixel connected to the samevertical signal line 53. Thereafter, the drain line 57 is made low, toapply a rest pulse (RST). Thereupon, the floating diffusion 38 returnsto the low.

FIGS. 18B and 18C are potential diagrams of the same. FIG. 18B is thecase that the reset transistor 36 (shown “RST” in the figure) is not adepression type, wherein the initial low level is to be determined bythe low level on the drain line 57. In a non-selected period of oneframe, electrons gradually leak from the floating diffusion 38 (shown at“FD” in the figure) to the drain line 57 (shown at “DRN” in the figure),causing a voltage rise. Meanwhile, the floating diffusion 38 of aselected pixel has a high level determined by a channel voltage Vch (on)of upon turning on the reset transistor 36. Specifically, it assumes asomewhat low value. Receiving photoelectrons in that state, it isswitched over to the low. Accordingly, the floating diffusion 38 has adynamic range as shown in the figure, excepting the margin.

FIG. 18C is the case that the reset transistor 36 is a depression type.The channel voltage Vch shifts downward in the figure, correspondinglyto a low threshold. However, in case the reset pulse RST has the sameamplitude, the amplitude ΔVch of which is the same as that of FIG.18(B). The initial low level is determined by the channel voltage Vch(off) of upon turning off the reset transistor 36. In FIG. 18B, there isa difference by a threshold amount between Vch (off) and initial lowlevel. However, it does not exist in FIG. 18C.

In that state, electrons gradually leak. However, when the pixel on thenext row is selected and the drain line 57 is switched over to the low,because the reset transistor 36 even if turned off is of the depressiontype, the floating diffusion 38 of the foregoing pixel again returns tothe initial low level. Each time the processing proceeds row by row, thepotential at the floating diffusion 38 returns to the initial low level.Accordingly, there is less potential rise even immediately after eachframe.

From these reasons, as can be seen from FIG. 18C, in case the resettransistor 36 is made as a depression type, the dynamic range on thefloating diffusion 38 can be broadened, thus making it possible toimprove the problem of dynamic range reduction unique to the unit pixel3 having a three-transistor structure having no vertical selecttransistor 40.

In this connection, by further deepening the degree of depressionthrough the utilization of the margin, Vch (on) can be set higher thanthe high level on the drain line 57. Namely, the reset transistor 36 ismade in such a deep depression type that can reset the floatingdiffusion 38 to the DRN-voltage high level when power voltage issupplied to the gate. Thereafter, in case the reset transistor 36 isturned off, the high level at the floating diffusion 38 is lowered by anamount of a contribution of field-though, etc. from the DRN-voltage highlevel. In this case, there is obtained a merit that the high level atthe floating diffusion 38 is determined by the high level on the drainline 57 whereby the threshold variation of the reset transistor 36 isnot put on the pixel output.

In the above, although the present invention was explained by using theembodiments, the technical scope of the invention is not to be limitedby the scope as described in the embodiments. The foregoing embodimentscan be modified or improved within a range not departing from the gistof the invention. The forms thus modified or improved are includedwithin the technical scope of the invention.

Meanwhile, the foregoing embodiments are not to limit the inventions ofthe claims. Furthermore, all the combinations of the features explainedin the embodiments are not necessarily required for the resolution meansof the invention. The foregoing embodiments include the inventions invarious phases. Various inventions can be extracted by a propercombination of a plurality of constituent elements disclosed. Even incase some constituent elements are deleted from all the constituentelements shown in the embodiment, the structure deleted of the someconstituent elements is to be extracted as an invention as long aseffect is available.

For example, the improvement technique based on the first approachshowed the arrangement example that can increase the transition time ofupon turning off the drain-line voltage even when supplied with a drivepulse having the usual pulse form to be inputted to the DRN drive buffer140. The concrete means for achieving this applied the technique thatmakes proper the W/L ratio of the transistor constituting the drivebuffer or make proper the operation current during drive by the use ofcontrol resistances or current sources.

However, without limited to these arrangements, various controltechniques and arrangements can be used as long as voltage transitiontime is increased on the drain line, which are also included in thetechnical idea of the invention.

For example, the arrangement may be such that the unit pixels inthree-transistor structure similar to those of the conventional are usedon the device while the drive signal for input to the DRN drive buffer140 be not in a pulse form but blunted in its waveform to meet theforegoing condition. In order for this, between the timing generator forgenerating a pulse-formed drive signal and the device (vertical scanningcircuit 14 in the above example), a waveform shaping circuit ispreferably provided to blunt the pulse in a manner meeting the foregoingcondition. This can make even the characteristic at between theperipheral pixel and the center pixel similarly to the explanations inthe above embodiments, thus reducing saturation shading.

Meanwhile, the above embodiment explained as an example the sensorconfigured by the NMOS-structured unit pixels. However, this is notlimitative, i.e. those configured by the PMOS-structured pixels canenjoy the similar operation/effect to those explained in the aboveembodiment by reversing the potential relationship (reversing thepositive and negative of potential).

Meanwhile, the above embodiment, although exemplified the pixel havingone photodiode and three transistors, the present invention is notlimited to the embodiment, is similarly applied to the pixel principallysimilar in operation, e.g. each one of reset transistor and amplifiertransistor is shared for two photodiodes and two read selecttransistors.

Meanwhile, although the above noticed saturation shading empiricallygreatest in affection, it is apparent that, in case potential isdifficult in fluctuation at between the center and the periphery of thepixel region, the other characteristic than saturation signal amountpossesses shading. Blunting the fall time or rise time for driving thedrain line improves the other shading phenomenon than saturation signalamount because of reducing well potential fluctuation and brings it intoa uniform.

Meanwhile, the solid-state imaging device of the invention does notrefer only to the one-chip device but means a concept including a cameramodule and camera including an optical system and a signal processorchip.

As described above, the present invention provides an improvementtechnique based on a first approach for suppressing the saturationshading phenomenon in the solid-state imaging device having unit pixelsin a three-transistor structure. This applies a technique of makingproper the W/L ratio of a transistor structuring a drain drive buffer orof making proper the operation current during drive by use of a controlresistance or current source, thereby increasing the transition timeupon turning off the drain voltage longer, e.g. five times or greaterand ten thousand times or smaller, than the transition time in turningoff any of the reset line and transfer line. This makes it possible tomake uniform the characteristic of the pixels at the periphery and atthe center, and hence to reduce saturation shading amount.

Likewise, in the improvement technique based on the second approach forsuppressing the saturation shading phenomenon, internally or externallyprovided is a voltage source for making the transfer-gate low levelvoltage a voltage value in a magnitude sufficient for forming apotential barrier against charge leak from the charge generating sectionto the charge storing section. This can suppress the phenomenon thatsaturation voltage decreases at pixel-region center. As a result,saturation shading amount can be reduced.

Likewise, in the improvement technique based on the third approach forsuppressing the saturation shading phenomenon, a bias line isstructurally provided to fix the well potential. This can prevent thereduction of saturation electrons at the pixel-region center due towell-potential fluctuation, decreasing the amount of shading.

Meanwhile, in the technique for resolving the problem of dynamic rangereduction in the solid-state imaging device having unit pixels in athree-transistor structure (improvement technique based on the fourthapproach), the reset transistor of the reset section structuring theunit pixel is provided as a depression type. This can suppress the leakcurrent caused by the reset transistor. This can broaden the dynamicrange at the charge storing part. Thus, the dynamic range conventionallyrestricted at the floating diffusion (charge storing part) can bebroadened.

1. A solid-state imaging device comprising: unit pixels including: acharge generating section for generating a charge in an amountcorresponding to a light received, a charge storing part for storing acharge generated by the charge generating section, a transfer gatesection arranged between the charge generating section and the chargestoring part for transferring the signal charge generated by the chargegenerating section to the charge storing part, a pixel signal generatingsection for generating a pixel signal corresponding to the signal chargestored in the charge storing part, and a reset section for resetting alevel of the charge storing part; a transfer line connected commonlywith other unit pixels and connected to the transfer gate section; atransfer drive buffer for driving the transfer line; a reset lineconnected commonly with other unit pixels and connected to the resetsection; a reset drive buffer for driving the reset line; a drain lineconnected commonly with other unit pixels and connected to the resetsection and the pixel signal generating section; a drain drive bufferfor driving the drain line; and a signal line for receiving the pixelsignal generated by the pixel signal generating section and connectedcommonly with other unit pixels; whereby a pixel select operation, foroutputting the pixel signal generated by the pixel signal generatingsection to the signal line, is carried out under potential control atthe charge storing part; and wherein an off transition time of a voltagewaveform on the drain line when driven by the drain drive buffer, isfive times or greater and ten thousand times or smaller relative to anoff transition time on any of the signals applied to the reset line whendriven by the reset drive buffer and the transfer line when driven bythe transfer driven buffer.
 2. A solid-state imaging device according toclaim 1, wherein the off transition time of the signal applied to thedrain line is in a range of 50 to 600 times relative to an offtransition time of a signal applied to the reset line or the transferline.
 3. A solid-state imaging device according to claim 1, furthercomprising a pixel region arranged with the unit pixels in atwo-dimensional matrix form, having a display resolution on the pixelregion conforming to a VGA rating, wherein the off transition time of asignal applied to the drain line is 10 nanoseconds or greater and 1000nanoseconds or smaller.
 4. A solid-state imaging device according toclaim 3, wherein the off transition time of a signal applied to thedrain line is 40 nanoseconds or greater and 600 nanoseconds or smaller.5. A solid-state imaging device according to claim 4, wherein the offtransition time of a signal applied to the drain line is 170 nanosecondsor greater.
 6. A solid-state imaging device according to claim 1,wherein the off transition time of a signal applied to the drain line isa half of a pixel clock period or greater and an off-period or smaller.7. A solid-state imaging device according to claim 1, wherein thetransfer drive buffer includes a transistor connected at least to thetransfer line, the reset drive buffer includes a transistor connected atleast to the reset line, and the drain drive buffer includes atransistor connected at least to the drain line, wherein the transistorconnected to the drain line has a W/L ratio (W is a gate width, L is agate length) set in a range of 1/5 times to 1/2500 times greater thanany of a W/L ratio of the transistor connected to the transfer line anda W/L ratio of the transistor connected to the reset line.
 8. Asolid-state imaging device according to claim 7, wherein the transistorconnected to the drain line has a W/L ratio set in a range of 1/10 timesto 1/500 times greater than any of a W/L ratio of the transistorconnected to the transfer line and a W/L ratio of the transistorconnected to the reset line.
 9. A solid-state imaging device accordingto claim 1, further comprising a resistance element for limiting a drivecurrent provided between an off-side reference line of the drain drivebuffer and a reference power source.
 10. A solid-state imaging deviceaccording to claim 1, further comprising a current source for regulatinga drive current provided between an off-side reference line of the draindrive buffer and a reference power source.
 11. A solid-state imagingdevice comprising: unit pixels including: a charge generating sectionfor generating a signal charge in an amount corresponding to a lightreceived, a charge storing part for storing a charge generated by thecharge generating section, a transfer gate section arranged between thecharge generating section and the charge storing part for transferringthe signal charge generated by the charge generating section, a pixelsignal generating section for generating a pixel signal corresponding tothe signal charge stored in the charge storing part, and a reset sectionfor resetting the signal charge stored in the charge storing part; atransfer line connected commonly with other unit pixels and connected tothe transfer gate section; a transfer drive buffer for driving thetransfer line; a reset line connected commonly with other unit pixelsand connected to the reset section; a reset drive buffer for driving thereset line; a drain line connected commonly with other unit pixels andconnected to the reset section and the pixel signal generating section;a drain drive buffer for driving the drain line; and a signal line forreceiving the pixel signal generated by the pixel signal generatingsection and connected commonly with other unit pixels; whereinoutputting the pixel signal generated by the pixel signal generatingsection to the signal line is carried out under potential control at thecharge storing part; and wherein an off transition time, of a voltagewaveform applied to the drain line when driven by the drain drive bufferis longer than an off transition time on any of the reset line whendriven by the reset drive buffer and the transfer line when driven bythe transfer driven buffer.
 12. A solid-state imaging device comprising:a solid-state imaging element having unit pixels including: a chargegenerating section for generating a signal charge in an amountcorresponding to a light received, a charge storing part for storing acharge generated by the charge generating section, a transfer gatesection arranged between the charge generating section and the chargestoring part for transferring the signal charge generated by the chargegenerating section, a pixel signal generating section for generating apixel signal corresponding to the signal charge stored in the chargestoring part, and a reset section for resetting the signal charge storedin the charge storing part; a transfer line connected commonly withother unit pixels and connected to the transfer gate section; a resetline connected commonly with other unit pixels and connected to thereset section; a drain line connected commonly with other unit pixelsand connected to the reset section and the pixel signal generatingsection; and a signal line for receiving the pixel signal generated bythe pixel signal generating section and connected commonly with otherunit pixels; whereby a pixel select operation, for outputting the pixelsignal generated by the pixel signal generating section to the signalline, is carried out under potential control at the charge storing part;and a waveform shaping section for receiving a drive pulse for drivingthe drain line and carrying out a waveform shaping such that an offtransition time for a voltage waveform applied to the drain line, islonger than an off transition time of a voltage waveform applied to thereset line and the transfer line.
 13. A solid-state imaging deviceaccording to claim 12, wherein the waveform shaping section carries outa waveform shaping such that an off transition time for a voltagewaveform applied to the drain line is five times or greater and tenthousand times or smaller than the off transition time for signalsapplied to the reset line and the transfer line.
 14. A drive controlmethod for a solid-state imaging device comprising: unit pixelsincluding: a charge generating section for generating a signal charge inan amount corresponding to a light received, a charge storing part forstoring a charge generated by the charge generating section, a transfergate section arranged between the charge generating section and thecharge storing part for transferring the signal charge generated by thecharge signal generating section to the charge storing part, a pixelsignal generating section for generating a pixel signal corresponding tothe signal charge stored at the charge storing part, and a reset sectionfor resetting the signal charge stored in the charge storing part; atransfer line connected commonly with other unit pixels and connected tothe transfer gate section; a reset line connected commonly with otherunit pixels and connected to the reset section; a drain line connectedcommonly with other unit pixels and connected to the reset section andthe pixel signal generating section; and a signal line for receiving thepixel signal generated by the pixel signal generating section andconnected commonly with other unit pixels; whereby a pixel selectoperation, for outputting the pixel signal generated by the pixel signalgenerating section to the signal line, is carried out under potentialcontrol at the charge storing part, the drive control methodcharacterized in that: the drain line is driven such that an offtransition time applied to the drain line is longer than an offtransition time of a waveform applied to any of the reset line and thetransfer line.
 15. A drive control method according to claim 14, whereinthe drain line is driven such that an off transition time of a voltagewaveform applied to the drain line is five times or greater and tenthousand times or smaller than the off transition time on both the resetline and the transfer line.
 16. A solid-state imaging device comprising:an imaging region arranged with a plurality of pixels; and a circuitregion for supplying a drive pulse to the imaging region; the pixelhaving: a photoelectric converting section for generating a chargecorresponding to an amount of incident light, a charge storing part forstoring a charge read from the photoelectric converting section, and areset section for resetting the charge stored at the charge storingpart; wherein the transfer gate is connected with a transfer line, thereset section is connected with a reset line, and the charge storingpart is connected with a drain line; the circuit region supplying afirst pulse to the drain line, a second pulse to the reset line, and athird pulse to the transfer line; the first pulse having a waveformlonger in off transition time than a waveform of the second pulse andthird pulse.